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Climate Data

Hurrell wintertime SLP-based Northern Annular Mode (NAM) Index

The NAM (or Arctic Oscillation) is defined as the first EOF of NH (20°-90°N) winter SLP data. It explains 23% of the extended winter mean (December-March) variance, and it is clearly dominated by the NAO structure in the Atlantic sector. Although there are some subtle differences from the NAO regional pattern over the Atlantic and Arctic (see Figure 1 below), the main difference is larger amplitude anomalies over the North Pacific of the same sign as those over the Atlantic. This feature gives the NAM a more annular (or zonally-symmetric) structure. Positive values of the NAM are associated with lower-than-normal sea level pressures over the Arctic and westerly wind anomalies along ~55º-60ºN.

Key Strengths:

Long NAM Index extending back to 1899

Characterizes changes in the position and strength of the NH mid-latitude jet

Key Limitations:

Physical distinction from NAO and North Pacific indices is not completely agreed upon by researchers

Dependent on any inherent weaknesses in the source data set and its gridding scheme

Technical Notes

Observational data is subject to corrections after it has been published and used in our indices. Once noted, these corrections are applied to our input data, and the corresponding climate indices may change.As is the nature of PC-based indices, every time additional data is
used to compute the EOF the individual PC values will likely change. It
is thus recommended that one downloads an entire climate index each time
they wish to update their holdings.

Citation:

Notes:

The DJFM PC index value for year N refers to an average of December year N-1 and January, February, and March year N SLP values prior to the EOF calculation. (Example: The 1999 PC value was based on the average of December 1998 and January, February, and March 1999 SLP values.)

Key Figures

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Thumbnails

Captions

The NAM pattern (top) explains ~24% of the extended winter-mean (DJFM) variance, and is clearly dominated by the NAO structure in the Atlantic sector. Although there are some subtle differences from the regional pattern of the NAO (EOF1 of DJFM North Atlantic SLP; bottom) over the Atlantic and Arctic, the main difference is larger amplitude anomalies over the North Pacific of the same sign as those over the Atlantic. This feature gives the NAM a more annular (or zonally-symmetric) structure. (Climate Data Guide; A. Phillips)

Time series of the various NAO and NAM indices. They are highly correlated. (Climate Data Guide; A. Phillips)

Acknowledgement of any material taken from this page is appreciated. On behalf of experts who have contributed data, advice, and/or figures, please cite their work as well.

Hurrell wintertime SLP-based Northern Annular Mode (NAM) Index

The NAM (or Arctic Oscillation) is defined as the first EOF of NH (20°-90°N) winter SLP data. It explains 23% of the extended winter mean (December-March) variance, and it is clearly dominated by the NAO structure in the Atlantic sector. Although there are some subtle differences from the NAO regional pattern over the Atlantic and Arctic (see Figure 1 below), the main difference is larger amplitude anomalies over the North Pacific of the same sign as those over the Atlantic. This feature gives the NAM a more annular (or zonally-symmetric) structure. Positive values of the NAM are associated with lower-than-normal sea level pressures over the Arctic and westerly wind anomalies along ~55º-60ºN.

The National Center for Atmospheric Research is sponsored by the National Science Foundation. Any opinions, findings and conclusions or recommendations expressed in this material do not necessarily reflect the views of the National Science Foundation.